Phosphoprotein phosphatases will be purified from skeletal muscle and liver. Substrate specificity will be tested using P32-histone, phosphorylase, glycogen synthetase, and phosphorylase kinase. With skeletal muscle glycogen synthetase, the specificity of the phosphatase for the several different phosphorylated sites will be examined. The physical properties of the purified phosphatase will be determined. Regulation of the enzyme in vitro by a variety of ligands will be examined. An investigation will be made of the reported interaction of an inhibitory protein with the phosphatase. BIBLIOGRAPHIC REFERENCES: Soderling, T.R. (1975): Regulation of glycogen synthetase: Specificity and stoichiometry of phosphorylation of the skeletal muscle enzyme by cyclic 3':5'-AMP-dependent protein kinase. J. Biol. Chem., 250: 5407-5412. Soderling, T.R. (1976): Regulation of glycogen synthetase: Effects of trypsin on the structure, activity, and phosphorylation of the skeletal muscle enzyme. J. Biol. Chem., in press.